The present invention relates to a multilayered polyamide film with improved resistance to pinholes, particularly to a multilayered polyamide film which maintains a stable pitch in the running direction during printing or lamination and is well suited for lamination.
Conventionally, multilayer films containing a polyamide resin have been widely used in various fields because of their gas barrier properties, toughness, etc. The market requires further improvement of film toughness, especially the resistance to pinholes.
To realize further improvement in pinhole resistance, however, the polyamide resin layer needs to be softened. Softening of the polyamide resin layer tends to cause the film to stretch in the running direction (the machine direction) during printing or lamination. Consequently, a problem arises regarding pitch accuracy in the machine direction.
To solve the problem of the prior art, the inventors focused on the relationship between the softness of the film and the amount of stretch in the machine direction, and achieved the present invention. A primary object of the present invention is to improve pinhole resistance and pitch accuracy in the machine direction, thus achieving improved gas barrier properties and more stable processability compared to the prior art.
The present invention is characterized by a multilayered polyamide film comprising at least two polyamide resin layers, the film having excellent processability such that the film develops less than 10 pinholes when evaluated for pinhole resistance by flexing it 1000 times at 25xc2x0 C., and has an elongation of 6 mm or less at 120xc2x0 C. in pitch accuracy evaluation in the machine direction.
The multilayered polyamide film of the invention comprises at least three layers comprising in order of a polyamide resin layer, a saponified ethylene-vinyl acetate copolymer layer and a polyamide resin layer, or at least three layers comprising in order of a polyamide resin layer, a xylylene diamine polyamide resin layer and a polyamide resin layer.
The multilayered polyamide film of the invention comprises at least three layers and may have, for example, a three-layer, five-layer or seven-layer structure. Examples of five-layer structure films include a film consisting of a polyamide resin layer, a saponified ethylene-vinyl acetate copolymer layer, a polyamide resin layer, a modified polyolefin adhesive resin layer and a linear low density polyethylene, and a film consisting of a polyamide resin layer, a xylylene diamine polyamide resin layer, a polyamide resin layer, a modified polyolefin adhesive resin layer and a linear low density polyethylene. Examples of seven-layer structure films include a film consisting of a linear low density polyethylene, a modified polyolefin adhesive resin layer, a polyamide resin layer, a saponified ethylene-vinyl acetate copolymer layer, a polyamide resin layer, a modified polyolefin adhesive resin layer and a linear low density polyethylene, and a film consisting of a linear low density polyethylene, a modified polyolefin adhesive resin layer, a polyamide resin layer, a xylylene diamine polyamide resin layer, a polyamide resin layer, a modified polyolefin adhesive resin layer and a linear low density polyethylene.
There is no specific limitation on the polyamide for forming the polyamide resin layers of the invention. Examples of useful polyamides include nylon 6, nylon 66, nylon 12 and copolymers thereof, nylon 6T/6I, nylon MXD-6 and the like. These polyamides may be used in combinations of two or more. Further, in order to increase the pinhole resistance of the film, a modified ethylene-vinyl acetate copolymer may be added as a component for giving flexibility to the film.
The polyamide resin layer preferably comprises 99 to 85 wt. % of a polyamide and 1 to 15 wt. % of a modified ethylene-vinyl acetate copolymer, more preferably 99 to 90 wt. % of the former and 1 to 10 wt. % of the latter, particularly 97 to 93 wt. % of the former and 3 to 7 wt. % of the latter.
Examples of modified ethylene-vinyl acetate copolymers include (1) resins with partially saponified xe2x80x94OCOCH3, (2) resins produced by partially substituting xe2x80x94OCOCH2CH3 for xe2x80x94OCOCH3 and (3) resins resulting from partial graft polymerization of an acid anhydride such as maleic anhydride.
There is no specific limitation on the saponified ethylene-vinyl acetate copolymer. Examples of saponified ethylene-vinyl acetate copolymers include copolymers with an ethylene content of about 20 to 65 mole %, preferably about 29 to 44 mole %, and with a saponification degree of about 90% or higher, preferably about 95% or higher. There is no specific limitation on the xylylene diamine polyamide resin. Examples of useful xylylene diamine polyamide resins include polymers synthesized from m- and/or p-xylylene diamine and a dicarboxylic acid such as adipic acid.
The film of the invention may contain different kinds of polymers as long as they do adversely affect the object of the invention and may contain organic additives such as antioxidants, thermal stabilizers, lubricants, UV absorbents and the like in typical amounts.
The total thickness of the film of the invention is about 10-40 xcexcm, preferably about 12-25 xcexcm. The thickness of each of the polyamide resin layers in the film, of which there are at least two, is about 3-15 xcexcm, preferably about 5-10 xcexcm. The thickness of the saponified ethylene-vinyl acetate copolymer layer is about 2-10 xcexcm, preferably about 3-10 xcexcm. The thickness of the xylylene diamine polyamide resin layer is about 2-10 xcexcm, preferably about 3-10 xcexcm.
The flat polyamide multilayer film of the invention can be obtained, for example, by co-extruding the resin layers from a T-die and superposing the layers in an appropriate order onto a chilled roll where cooling water is circulating. The film thus obtained is stretched to 2 to 4 times its original size in the machine direction, for example, at 50 to 100xc2x0 C. using a roll stretching machine. Then the film is stretched to 2 to 5 times its original size in the transverse direction at an atmospheric temperature of 90 to 150xc2x0 C. using a tenter stretching machine. Subsequently, the film is thermally treated at an atmospheric temperature of 80 to 220xc2x0 C. using the tenter stretching machine. The multilayer film of the invention may be subjected to monoaxial stretching or biaxial stretching (simultaneous or sequential). The multilayer film may be treated with corona discharge surface treatment on one or both sides, if necessary.
With regard to pinhole resistance, the multilayer film of the invention develops less than 10 pinholes when evaluated for pinhole resistance by flexing it 1,000 times at 25xc2x0 C. Preferably, the number of pinholes is 6 or less, more preferably 2 or less. Ten or more pinholes indicate no substantial improvement in pinhole resistance. If products are packaged with such film, pinholes tend to occur during actual transportation.
The multilayer film of the invention has an elongation of 6 mm or less at 120xc2x0 C. in vertical pitch evaluation. Preferably, the amount of elongation is 5 mm or less. When the amount of elongation is 6 mm or less in vertical pitch evaluation, the film maintains a stable pitch in the machine direction during actual printing and lamination, so that no problems will arise in the post-processing process of forming the film into a bag. By contrast, when the amount of elongation is more than 6 mm in vertical pitch evaluation, the film is prone to stretch due to the tension applied during printing or lamination, resulting in a large deviation and variation from the normal pitch. Such low pitch accuracy causes problems in the post-processing process of forming the film into a bag.
The temperature of 120xc2x0 C. was chosen for the vertical pitch evaluation because this is the maximum temperature to which films are usually exposed during actual printing and lamination.
The multilayer film of the invention has high toughness and excellent resistance to pinholes and is thus suitable for packaging heavy items, especially rice cakes, liquid items such as soups and the like.
The present invention will be described below in more detail with reference to Examples and Comparative Examples. The following methods were used to measure the characteristic properties of the invention.
Pinhole Resistance Evaluation
Pinhole resistance was evaluated using a Gelbo flex tester manufactured by Rikagaku Kogyo K.K., in the following manner. Each sample film was formed into a cylindrical bag 150 mm in width when laid flat and 300 mm long. The bag was attached to the Gelbo flex tester and flexed 1,000 times at 25xc2x0 C. at a torsional angle of 440xc2x0 and with a twist and linear motion of 15.0 cm. Then, using a penetrant, the number of pinholes formed on the central portion of the sample (measuring area: 300 cm2) was counted.
Vertical Pitch Evaluation
The vertical pitch was evaluated in the following manner. Each sample was cut into long strips. Then 600 g of load was applied to the strip at an atmospheric temperature of 120xc2x0 C. to measure the amount of elongation of a 40 mm line drawn on the central portion of the sample.